1. Field of the Invention
The present invention relates to an image processing method for processing an image in the form of digital signals and an apparatus therefor, and more particularly to an image processing method for pseudo intermediate tone reproduction by digitization of input image and an apparatus therefor.
2. Related Background Art
There are already known printers using a binary recording method, forming a record by printing dots or not, such as laser beam printer (LBP) or ink jet printer. For reproducing an image with intermediate density such as a photograph or a screentone original with a copying apparatus employing such a binary printer, the image data read from such original with intermediate tone are processed by an image processing circuit for pseudo intermediate tone reproduction.
The so-called dither method is widely used as one of such pseudo intermediate tone processing methods.
Said dither method is advantageous in that it is capable of said pseudo intermediate tone reproduction with a simple hardware structure and with a low cost, but is associated with following drawbacks:
(1) a periodic fringe pattern (Moire fringe pattern) is formed on the reproduced image when the original is a screentone image such as a printed image, thus deteriorating the image quality; and
(2) when the original contains linetone images or characters, the image quality is deteriorated as the lines are not satisfactorily reproduced.
The drawback (1) can be reduced by a smoothing method (spatial filtering) applied to the read intermediate tone image data, while the drawback (2) can be reduced for example by edge enhancement, but it is difficult to obtain satisfactory reproducibility for various images such as a photograph, an image, a linetone image and characters. Also such processes require a complicated circuitry, deteriorating the inherent advantage of the dither method.
Based on these backgrounds, developments are being made on a so-called error dispersion method as one of such pseudo intermediate tone processing methods.
The error dispersion method, disclosed for example by R. S. Floyd and L. Steinberg in "An Adaptive Algorithm for Spatial Grey Scale", SID 75 Digest, is characterized, in binary digitizing of input image data, by dispersing the density error between the input image data and the output image data into surrounding pixels, thereby conserving the original image density.
Such error dispersion method is superior to the aforementioned dither method in tone reproduction and resolution, but is associated with drawbacks of formation of specific stripe patterns in the uniform density area of the image and of granular noises in the highlight area of the image due to scattered dot formation.
For avoiding such drawbacks, various methods have been proposed for example in the U.S. patent applications Ser. Nos. 137,439, 140,029 and 145,593 and a U.S. patent application filed on May 9, 1988, corresponding to the Japanese Patent Application Nos. 62-121611, 62-121612 and 62-121613.
Such error dispersion method is satisfactorily usable in a copying apparatus utilizing a raster scanning, but will give rise to a following drawback when employed in a serial scanning. A copying apparatus employing such serial scanning is disclosed in the U.S. patent application Ser. No. 798,672.
In serial scanning, the image is read in succession at first in an area (a) and then in an area (b), and is subjected to processing by the error dispersion method. The errors obtained in processing the image of the area (a) are lost when the processing proceeds to the area (b). More specifically, in binary digitizing of the area (b), no carry-over errors from the area (a) are available, so that appropriate binary digitizing of the area (b) cannot be achieved. Consequently a discontinuity appears between the processing of the area (a) and that of the area (b), thus giving rise to a streak or a black line on the boundary.
Also the binary digitizing of 255th and 256th pixels of each line in the area (a) requires error information generated in the binary digitizing of the 1st and 2nd pixels in each line of the area (b). Proper binary digitizing cannot be achieved due to the lack of such error information at the boundary of the areas (a) and (b), and a streak is formed on said boundary.
Now, a further explanation will be provided of the streak formation on the boundary of the processing areas.
Let us consider a case of utilizing a 3.times.5 error dispersion matrix shown in FIG. 2A, in which numerals indicate an example of error distribution ratio.
In FIG. 2B it is assumed that each main scanning line has 256 pixels, and that a notation a(255, 2) indicates the 255th pixel in the 2nd scanning line, in the sub scanning direction, of the area (a).
At first let us consider the binary digitizing in an object pixel a(255, 1). The errors generated in the binary digitizing of said pixel are added, as will be apparent from the dispersion matrix shown in FIG. 2A, to pixels b(1, 1), b(1, 2) and b(1, 3).
Also the errors generated in the binary digitizing of a pixel a(256, 1) are added to pixels b(1, 1), b(1, 2), b(1, 3), b(2, 1), b(2, 2) and b(2, 3). Similarly the errors generated in the 255th and 256th pixels in each line of the area (a) are added to the 1st and 2nd pixels of the lines in the area (b).
Then let us consider the binary digitizing of an object pixel b(1, 1). The errors generated in the binary digitizing of said pixel b(1, 1) are added, as will be apparent from FIG. 2A, to pixels a(255, 2), a(256, 2) and a(256, 3).
Also the errors generated in a pixel b(2, 1) are added to pixels a(256, 2) and a(256, 3). Similarly the erros generated in the 1st and 2nd pixels in each line of the area (b) are added to the 255th and 256th pixels of the lines in the area (a).
Therefore, if the error dispersion method is conducted without consideration of the boundary between the areas (a) and (b) as shown in FIG. 1A, the errors generated in the binary digitizing of the 1st and 2nd pixels in each line of the area (b) are not made available in the binary digitizing of the 255th and 256th pixels in each line of the area (a), so that said digitizing of the 255th and 256th pixels cannot be achieved in proper manner. Also the errors generated in the 255th and 256th pixels in each line of the area (a) have to be added to the 1st and 2nd pixels in the lines of the area (b). Thus, the binary digitizing of said 1st and 2nd pixels cannot be achieved in a proper manner unless said errors of the area (a) are retained until the processing of the area (b). In this manner a streak is generated at the boundary of the areas (a) and (b).